Liquid metal embrittlement
Cracking in a base metal caused by molten metal penetrating grain boundaries while under tensile stress.
Liquid zinc or copper can penetrate the steel grain boundaries at welding temperature and embrittle them. If the component is under tensile stress, it will crack along these grain boundaries – this is called liquid metal embrittlement. The steel itself does not melt, it is a pure surface effect. When welding galvanized sheet, you must grind off the zinc layer in the weld area beforehand. Otherwise, the molten zinc will wick into the hot steel and you'll get fine cracks next to the weld. Weld with low heat and don't let the weld cool under restraint, then this usually won't happen. Copper backing bars or copper-containing fillers can also cause such an effect, so be careful with them.